Apparatus and methods for introducing a force into a mechanical structure or structural system consisting of one or more rigid bodies or flexible elements have hereto for been known and/or utilized. It would be advantageous for the purposes of active control and/or damping of the structure or structural system when subjected to random and/or unwanted excitation forces, or for the purpose of structural identification (i.e., determination of the dynamic characteristics which make a particular structure unique such as its mode shapes and frequencies) for such introduced forces to be capable of following a predetermined variation in time which may either be harmonic or non-harmonic in nature. It is furthermore advantageous, both from the control and identification viewpoint, for the force-time history imparted to the structure to be capable of being varied at an extremely rapid rate while still maintaining "intelligence", that is, a pre-determined, analytically definable shape.
For example, it has been shown that it is possible to recover the dynamic characteristics of a structure with greater reliability, and to a higher degree of precision, if the structure can be excited with a force-time pulse which is analytically defined by an inverse gaussian distribution (see Carasso, A. S., and Simiu, E., "Estimation of Dynamic Green's Functions For Large Space Structures By Pulse Probing and Deconvolution"). Furthermore, the ability to load the test structure with as short (time duration) an inverse gaussian pulse as possible will enable the capture of a higher number of mode shapes, and therefore yield more precise knowledge of the dynamic characteristics of the structure. Such precision and speed of response of the force generation system is of particular use in the control of spacecraft and orbiting structures and for the control of flexible robotic systems, although the technique is equally applicable to, for example, the active control of a tall building subjected to earthquake loads.
It is a required characteristic of most such control/identification loading systems that they be capable of operating without the presence of a reaction surface, frame, or other interacting structure. This requirement is most obvious for the case of the control of spacecraft and orbiting structural systems.
Existing controlled-force generation technology includes closed loop servo-hydraulic and servo-pneumatic loading equipment now in common use for structural testing and industrial fabrication. Such equipment works on the principle of directing a pressurized fluid (either hydraulic oil or compressed air) from a pressurized reservoir to a bi-directional actuator which may be made to either extend, contract, or remain stationary depending upon state of an electronically controlled servovalve.
While it is possible to program these actuators to impart an "intelligent" forcing function into a test structure, the time for these actuators to respond to an instantaneous change in the command signal is generally about 0.020 second or longer. If it is assumed (for the sake of later comparison) that a continuous programmed, or "intelligent", forcing function requires 100 defining grid points, then the shortest possible programmed impulse that could be imparted to a test structure using such loading equipment would be 2 seconds or longer. In addition, such actuators require a reaction surface in order to be used.
Alternatively, several loading systems presently exist for spacecraft which require no reaction surface. These fall into a broad classification known as Reaction Control Systems (RCS). Of particular interest to the present discussion are those reaction control systems which comprise small rocket thrusters which may be used for translational as well as rotational control of spacecraft and orbiting structural systems.
Classical RCS thrusters employ a pressurized supply of fuel, an electronically controlled solenoid-type valve, a combustion chamber (if the thruster is of a bipropellant variety) and an expansion nozzle. The control solenoids are normally closed in such systems, and only the duration of the open time is subject to variation. Therefore, while solenoid controlled RCS thrusters can impart differing levels of thrust duration, there is no variation in thrust level (which is controlled by the fixed diameter of the thruster nozzle). Such systems also suffer from a lack of precise impulse bit repeatability due to the opening and closing characteristics of the valves, manifold fill times, and chemical ignition delay times.
A second type of RCS thruster employs a piezoelectric pump for the two fuel components of a bipropellant thruster (see, for example, Kattchee, N., "Piezoelectric Injection System For Vernier Impulse Thrusters," June 1967). This is known as the pulse-pumped vernier engine concept and operates on the principle of pulsing a piezoelectric stack with a specified voltage waveform. The piezoelectric stack is connected to a series of inlet and outlet valves which permit the pump to draw propellant into a holding chamber during a contraction of the stack and to expel the propellant through the outlet valve during an expansion of the stack.
Each such cycle delivers a finite, measurable quantity of fuel to a combustion chamber and subsequently to an expansion nozzle. The frequency of the arrival of the electrical pulses which drive the piezoelectric stack thus determines the total impulse delivered by the thruster during a specified length of time. While this is a useful method for metering precise impulses, the fastest response time thus far achieved has been on the order of 0.01 second, and thus a 100 point "intelligent" load pulse would be at least 1 second in duration, which is too long for accurate identification of higher frequency mode shapes that are presently of interest for spacecraft involving precise pointing requirements. Furthermore, the amplitude (peak force) of the thrust which can be achieved in this manner is severely limited by the maximum stroke of the piezoelectric stack, and thus amplitude modulation to achieve an "intelligent" force-time pulse is not practicable with this approach.
A variety of other piezoelectric valving systems have been heretofore suggested for use in a variety of applications (see, for example, U.S. Pat. Nos. 4,669,660, 5,029,610, 4,431,136, 5,025,766, and 3,055,631), some including amplification of the movement of the piezoelectric device (see, for example, U.S. Pat. No. 4,593,658). These systems, however, suffer many of the same impediments hereinabove noted and/or could not suitably perform functions applicable to the systems and problems addressed by the instant invention.